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Aviation History
1918
1918 - 1355.PDF
at right angles, will be spoken of as crossed. To fix ideas, suppose that in the block A the stress P is applied horizontally, and the stress Q vertically. Then in the block B, P is applied vertically and Q horizontally. iNow the light traversing either beam is broken up into a horizontally polarised and a vertically polarised wave, ani thejjconditions of travel of these two waves are inter- changed in A and B. Thus if the horizontally polarised raybe accelerated with regard to the vertically polarised ray by an amount " ••" after traversing A, the vertically polarised raywill be accelerated with regard to the horizontally polarised ray by the same amount " r " after traversing Bj the netacceleration or retardation after traversing both A and B being zero. We thus get a black field if the two blocks areplaced between crossed Nicols. We may therefore ascertain the stress difference in a givenspecimen of material (the stresses acting in known directions) by placing in front of it a test piece of the same material andsame thickness to which we apply known stresses in the same directions until we find that the field becomes black. Whenthis is the case, the stress difference in the test piece and specimen are equal in magnitude and opposite in sign. 5) Appearances in a Non-Uniform Field. Neutral, Isochromatic and Isoclinic Lines Let us now pass to the case of the appearances presentedwhen a flat slab or plate under stress in its own plane is examined in this way between crossed Nicols. In such aslab the stresses will vary from point to point. At any given point, however, the stress system can be reduced totwo principal normal stresses (tensions or pressures) in directions at right angles—these directions being knownas the principal axes of stress at the point. Every point NOVEMBKR 28, 1918 for which the principal stress difference is zero will appearblack in the field and the locus of such points forms a black band or " neutral line." Generally on either side of theneutral line is a white area, in which the principal stress difference is slight. If the applied stress is sufficiently largeon going outwards from the neutral line a rainbow band is met, beginning with orange and going into red tint ofpassage and blue. This is the " first order " coloured band. Beyond it will be a yellowish space, and a " second order "coloured band, reddish towards the neutral line and green away from it. These bands are called "isochromatic"lines, or lines of equal colour. Along any one isochromatic line the principal stress difference in the material is constantin magnitude. A mapping of the isochromatic lines there- fore gives a picture of the distribution of stress differencethroughout the material. As yield and rupture of many materials depends largely upon the value of the stress dif-ference, such a picture gives immediate information as to the dangerous areas or fail points. These will necessarilybe those points where the colour bands of highest order make their appearance as the load is increased. The neutral line and the isochromatic lines, however, arenot the only bands visible in the field. So far, in the previous discussion of the appearances presented by a uniformly loadedblock, it has been assumed that the polarising axes of the two Nicols were not parallel to the directions of stress. Onthat assumption we have seen light was in general restored. If, however, it happens that the polarising axes of theNicols are parallel to the directions of stress, the light is never restored, for then, of the two waves in the stressedmaterial, one coincides with the wave transmitted by the first Nicol, and the other is absent. The stressed materialdoes not therefore depolarise the light, and the second Nicol stops it entirely. It follows, then, that all points of the slab at which thedirections of the principal axes of stress are parallel to the polarising axes of the Nicols will appear black in the field,hence one or more black bands or brushes corresponding to the locus of such points. Lines of this kind are called " iso-clinic lines," or lines of equal inclination. If the Nicols be rotated together, the isoclinic lines movein the field, but the isochromatic lines remain fixed. If the applied loads be increased in any given proportion, the dis-tribution remaining the same, the isochromatic lines crowd in upon the neutral line. The neutral line and the isocliniclines remain fixed. The isoclinic lines are most useful when we do not knowbeforehand from other considerations the directions of the principal stresses at a given point. These directions can thenbe easily determined by rotating the Nicols until the visible isoclinic line passes through the point in question. The posi-tion of the polarising axes of the Nicols then gives the solu- tion of the problem, and enables us to set our test pieces atthe right angle if we use the null method of measuring the stress difference. (To be continued.) THE JUNKER ARMOURED BIPLANE FOR some time past there have been rumours current of aGerman all-metal aeroplane in which, it was said, even the wing covering was of metal. It is not, however, until quiterecently that we have been able to obtain reliable information concerning this machine. What is left of the Junker biplane,as the machine in question is called, is now included among the many interesting exhibits at the Enemy Aircraft ViewRooms at the Agricultural Hall, Islington, where our repre- sentatives have been permitted to examine this machine indetail. Owing to the damaged condition of the specimen on viewit has not been possible to give, this week, moVe than a brief outline of the main characteristics of the machine. Laterwe hope to be able to describe it in more detail. The Junker armoured biplane is designed for use as atrench fighter, and in contradistinction to the A.E.G. armoured biplane it was evidently designed with this object in viewfrom the start, the armour not being attached, as a supple- ment, to an ordinary girder structure, as was the case withthe A.E.G., but forming the main fuselage structure at the same time as providing the armouring. The shape of the front portion of the fuselage may be seenin one of the accompanying photographs. It has flat sides of armour plating, and a slightly curved top of aluminium.The bottom of the body is formed of three fiat surfaces, the middle one of which is horizontal, the other two sloping soas to connect the edges of the horizontal bottom with those of the vertical sides. The engine, a 230 h.p. Benz, is also protected by armour plating, which is detachable so as toallow access to the engine. This is accomplished by hinging the armour at this point, the port and starboard engine armourbeing separately detachable. In the accompanying photo- graph the engine armour of one side will be seen lying on thefloor in front of the fuselage.. The armouring is finished off just behind the gunner's cockpit, where it is continued acrossthe fuselage by a curved armour plate shaped to form the gunner's back rest. Of the rear portion of the fuselage nothing remains on themachine examined, and nothing can therefore be said regard- ing the details of its construction. Judging from such evidenceas fittings for large diameter tubes at the four corners it would appear that the rear part of the fuselage has been built upof a tubular framework of Duralumin. The wings of the Junker are of great interest as being of aconstruction very dissimilar to any so far seen. The internal construction of the wings is in the-form of Duralumin, tubescrossing diagonally and connecting the tubular spars, which latter are far greater in number than is jjrdinarily found inan aeroplane wing. It has evidently been the aim of the designer to distribute the spars over the wing section ratherthan to provide two spars located in the usual manner. In the Junker there may be said to be six spars if one countsthe top and bottom tubes lying vertically above one another as one spar. The lower photograph will give some idea ofthe general distribution of the various wing members. In section the planes of the Junker are enormously deep,
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